As the flow rate sensor installed in an intake air passage of an internal combustion engine of an automobile for measuring flow rate of the intake air, a thermal type sensor is mainly used because it can detect in direct mass flow rate. A thermal measuring device is manufactured with a semiconductor ultra-fine processing technology on a semiconductor substrate like a silicon (Si) substrate. This technology is very economical because it can easily realize mass-production system. Moreover, attention is paid to this manufacturing technology because the manufacturing facility is rather small in size and can be driven with lower electric power.
As the flow rate measuring device, a flow rate sensor described in the patent document 1 is known. This sensor measures flow rate with a temperature sensor allocated in a thin film area formed on a substrate. Since this thin film area has small thermal capacity, thermal response can be obtained quickly in accordance with change of flow rate.
When a flow rate sensor using a flow rate measuring element which responds quickly is applied to the control of automobile, even if flow rate changes rapidly or pulsation is generated within the intake pipe, it is possible send response corresponding to change of flow rate. Therefore, even in the operating condition described above, more accurate flow rate of air can be measured than that of the flow rate sensor of slower response rate having larger thermal capacity.
The flow rate sensor described in the patent document 1 allocates temperature sensors in the upstream and downstream sides of a heater for the flowing direction and it is heated with a heater. When the air does not flow, the temperature sensors allocated in the upstream and downstream sides are set to almost equal temperature and temperature difference between these sensors is almost zero. When the air starts to flow, a temperature sensor allocated in the upstream side of heater is cooled, while a temperature sensor allocated in the downstream side is further heated by receiving the heat from the upstream side. Temperature difference is therefore generated between the temperature sensors provided in the upstream and downstream sides. Since this temperature difference varies depending on the flow rate of air, the flow rate of air can be detected from the amount of temperature difference. Moreover, since a resistance value of the temperature sensor changes depending on the temperature, a voltage signal can be obtained depending on the flow rate by utilizing change of resistance value.
Moreover, when the air flows in the reverse direction, the temperature sensor of this type allocated in the downstream side of the heater is cooled on the contrary to the case described above. Accordingly, the flow rate in the reverse direction can also be detected, realizing detection of the air in the flowing direction. Measurement of the flow rate of air can therefore be realized more accurately than that with the flow rate sensor not including a direction detecting means even under the operating condition that the air flows toward an air cleaner from an engine (reverse flow of fluid).
[Patent Document 1]
Japanese Unexamined Patent Publication No. 500490/1998 (FIG. 1, line 3 to 18 on page 5 of the Specification)
Intake air pulsation usually increases in the lower number of revolutions of a 4-cylinder engine and reverse flow has been often generated at a throttle angle near the full throttle angle. In the case of an engine which can realize complicated controls such as variation of opening or closing time of valve corresponding to the restriction on exhaust gas and realization of low fuel consumption or the like in the recent years, pulsation and reverse flow are generated under the higher number of revolutions and thereby amount of reverse flow tends to increase.
Meanwhile, a voltage output type flow rate sensor for automobile assigns a voltage range of 0 to 5 V used in a control unit to the predetermined range of the flow rate of the engine. In the case where reverse flow is also detected, the range of flow rates in both forward and reverse flowing directions must be assigned within this voltage range. Since the conventional flow rate detecting element detects flow rate with the identical principle for both forward and reverse flows, the forward flow characteristic becomes almost symmetrical to the reverse flow characteristic about the point where the flow rate is zero. For example, when the voltage region of 4 V in the forward flow range of 0 to 500 kg/h is necessary, the voltage range of 4 V in the reverse flow range of 0 to 500 kg/h is also required.
In the conventional automobile engine, since the reverse flow rate has been as small as about 50 kg/h or less, it has been possible to cover the required range of flow rate by setting, for example, the reverse flow region to 0 to 1 V and the forward flow range to 1 to 5 V.
However, since the reverse flow rate tends to increase recently, it is required to widen the voltage range to be assigned to the reverse flow region and therefore the range of flow rate assigned to the forward flow relatively becomes smaller. In this case, if the voltage signal can be read only in the step of 5 mV because of the performance of a microcomputer used in the control unit, amount of change of voltage becomes 5 mV or less even when the flow rate changes in the range of 1 to 3% in the low flow rate area. Accordingly, the control unit can recognize the flow-rate signal only with the accuracy of about 3% in the low flow-rate area.
Moreover, under the condition that pulsation becomes large in the engine and reverse flow is generated, waveform during generation of the reverse flow is no longer uniformed depending on the shape of the intake pipe. Uniformity of the reverse flow waveform is broken because of the reasons that (1) flow around the flow rate sensor when the pulsation is generated is disturbed when a flow rate sensor is inserted to the intake pipe, and (2) generation of reverse flow is not constant for each cylinder, etc. Accordingly, a flow rate error is generated between an average flow rate obtained within shorter time of every period of the pulsation waveform and an average flow rate obtained from longer time of several periods.
In the control unit, fuel injection amount of injector is determined based on outputs of the flow-rate sensors provided in the interval, for example, of 2 ms. As described above, when the flow-rate sensor output varies, control error due to such variation increases as the number of revolutions rises.